Reflection • Reflection is the process in which light

strikes a surface and bounces back off that surface. How it bounces off the surface depends on the Law of Reflection and the type of surface it hits.

The Law of Reflection

• States when an object bounces off a flat surface, the angle at which it hits the surface will be equal to the angel at which it bounces away.

• Light coming from a light source is called an incident ray and the light that bounces off the surface is called a reflected ray. A line that is perpendicular ( 90 degrees with the surface) to the plane mirror is called the normal line.

The angles are named this way:

• The angle at which the object hits the flat surface is called the angle of incidence (i).

• The angle at which it rebounds from the flat surface is called the angle of reflection (r).

• Both angles are measured from the path of the object to the normal line which is a line perpendicular to the surface at the point of impact.

Light Follows the Same Law of Reflection

• When light strikes a flat reflective surface it obeys the same laws of reflection.

• The light will strike a flat surface and bounce off of it in a straight line path.

Reflective diagram• Incident ray: the ray of light

which strikes the surface.• Reflected ray: the ray of light

which leaves the surface.• The normal: a line

perpendicular to the surface is imagined at the point of reflection.

• The incident angle: the angle between the incidence ray and normal.

• The reflected angle: the angle between the reflected ray and the normal line.

• The angle of incidence is equal to the angle of reflection.

Predictable Behaviour• An image is formed in a mirror because light

reflects off all points on the object being observed in all directions. The rays that reach your eye appear to be coming from a point behind the mirror. Because your brain knows that light travels in a straight line, it interprets the pattern of light that reaches your eye as an image of an object you are looking at.

• Look at figure 7.17 on page 223

• an image in a mirror is the same size as the object and appears to be the same distance from the mirror as the object. (only true for flat mirrors)

• Figure 7.18 on page 223

Smooth Surface

• If light hits a smooth surface, the light reflects at an opposite angle to the angle it hits.

• Smooth surfaces reflect light uniformly. This creates a clear image when light is reflected by a plane mirror.

Rough surface• If it hits a rough

surface, the light is scattered.

• THEREFORE:• Rough surfaces appear to

reflect light randomly, but this seemingly scattered light creates the image of the print on the page. Light hits the white paper and reflects in all directions (some of it reaching your eye). Since there is no pattern, your eye just sees white light. The ink on the paper absorbs the light and no light from the ink reaches your eye. Therefore your eye sees the letters in black ink. When light reflects off a rough surface.

• When light reflects off a rough surface, diffuse reflection occurs.

Using Reflections

• Reflectors help to make bicycles and cars visible at night. A reflector is made up of hundreds of tiny, flat reflecting surfaces arranged at 90o angles to one another. These small surfaces are packed side by side to make the reflector. When light strikes the reflector the light bounces off the tiny surfaces and bounces back toward the light source.

• Pool players use the law of reflection to improve their game. Like a light ray, a pool ball travels in a straight line. In a 'bank shot' the cue ball is bounced off the cushion at an angle which enables the player to hit the target ball. This angle is calculated as the angle of contact (with the cushion) is equal to the angle of impact (with the target).

• The Law of reflection states that:

First: the angle of incidence equals the angle of reflection. EG if the

angle of incidence is 30 degrees, then the angle of reflection is also

30 degrees.

Second: that the incident ray, the normal and the reflected ray are

all in the same plane.

Refraction

Refraction • Refraction is the process in

which light is bent, when it travels from one medium to another. Light bends because it changes speed when it moves through materials that have different densities. Light travels slower in materials that are more dense, because there are more particles. The bending of light makes the object's image appear to be in a different position than it really is.

So......• Refraction occurs when

light moves into a new material and starts to move at a different speed.

• E.g. Light passing from air to water

• The speed of the light ray changes upon changing substances. In almost every case the direction of the light ray

changes also.

In the picture below...• Light rays change speed as they

go from air into the glass they change speed again when they exit the glass and enter the water.

• The light rays will change speed again as they move from the water to the glass and to the air again.

• This causes the pencil to look distorted.

• Each time the speed changes the light changes direction slightly.

• If the ray hits the surface of the material at an angle, the ray will be bent.

• See how the pencil appears to be displaced from itself.

Around a Bend with Light

• The Law of Refraction states that:

when light travels from one medium, to a denser medium, the light will be bent

toward the normal

and when it exits the denser medium into a less dense medium it will bend away

from the normal.

The new direction of light is called the angle of refraction.

• Refraction can also occur when light travels through air at different temperatures, because warm air is less dense than cold air. The refraction of light through air is called a mirage.

The pools of water you see on a hot summer day are often caused by this effect, because the air closer to the ground is hotter than the air above it. As you approach these pools, they disappear - because they were never there.

Is that all there is to Light? • What happens when light strikes a

surface? ... Type of Behaviour

What happens to light striking a surface

Nature of surface

What else happens?

Absorption Changes into some other kind of energy. Energy Transformation

Occurs mostly on rough, dark, opaque surfaces

Some light is usually reflected off the surface.

Reflection Bounces off the surface and travels in a new direction

Occurs best when light hits a smooth, shiny surface.

Some light is absorbed

Refraction Travels through the surface often in a new direction

Occurs when light strikes a different, transparent medium

Some light is usually reflected off the surface